sql(mysql): stop reading DISABLE_SQL_AUTO_PIPELINING in can_pipeline

[robobun]

Problem

• With BUN_FEATURE_FLAG_DISABLE_SQL_AUTO_PIPELINING=1, a MySQL query that is not .simple() never settles and blocks its connection. The client writes COM_STMT_PREPARE and never COM_STMT_EXECUTE.
• The flag branch in MySQLRequestQueue::can_pipeline (src/sql_jsc/mysql/MySQLRequestQueue.rs:70) returns false in every state. It alone permits the write of the execute (MySQLQuery.rs:343).
• Regression in v1.2.22 (refactor(MySQL) #22619). No user has reported it.

Fix

• Delete the flag branch, the only MySQL reader of the flag.
• Since v1.2.22 each write clears is_ready_for_query, only a complete reply sets it, and can_pipeline requires it. So the adapter writes one command at a time, except for MySQL: query writes can bypass queued-but-unwritten requests in the native request queue #32005.
• Verified: test/js/sql/sql-mysql.test.ts, whose flag-set cases fail on main. Other suites: Notes.
• Self-reviewed: 11 concerns raised, 11 addressed. One Windows failure under CPU load has no proven cause (Notes).

Background

• In Bug Report: Segmentation Fault in Bun v1.2.19 #21403 a maintainer told a user to set the flag against a Postgres crash, so such processes exist. In Postgres it stops a write behind a query with no reply yet.
• Considered can_execute_query() || can_pipeline() at the call site: can_pipeline stays false under the flag. Considered pipelined_requests == 0 || !flag: always true outside MySQL: query writes can bypass queued-but-unwritten requests in the native request queue #32005.

Downsides

• Question for a maintainer: the docs say that MySQL pipelines queries. Since v1.2.22 it does not (Bun.SQL MySQL: the docs say that queries are pipelined, the adapter writes one command at a time since v1.2.22 #44027). Is one command at a time the contract? If not, the flag needs a MySQL reader.
• MySQL ignores the flag and Postgres honours it.
• No added cost: 1002 to 0 flag reads for 1001 prepared queries, .text unchanged.

Notes

Regression range. The script below, against MariaDB 11.8.6, with the flag set. Without the flag all three queries resolve on each version.

import { SQL } from "bun";
const sql = new SQL({ url: "mysql://root@127.0.0.1:3306/bun_sql_test", max: 1 });
await sql`SELECT 1 AS v`.simple();
await sql`SELECT ${1} AS v`;
await sql`SELECT 3 AS v`.simple();

Version	Second query	Third query
1.2.21	resolves	resolves
1.2.22	never settles	never settles
1.3.0	never settles	never settles
1.4.3-canary.1	never settles	never settles
this branch	resolves	resolves

1.2.21 had two writers of the execute. The call that starts a query wrote it when can_execute or connection.canPipeline(). advance() wrote it with no condition and read canPipeline() only to decide if it continues. #22619 replaced both with if (connection.canPipeline()).

What the flag did on MySQL. No release gave a process with the flag set a client that writes one command at a time. The mock never answers EXECUTE b, and three executes of a cached statement are queued together:

Client	Flag	Commands after the warm-up
1.2.21	unset	EXECUTE b, EXECUTE c, EXECUTE d in one write
1.2.21	set	EXECUTE b, then EXECUTE c in a second write
1.2.22, 1.3.0, main	unset	EXECUTE b
1.2.22, 1.3.0, main	set	none, the client hangs after PREPARE
this branch	unset and set	EXECUTE b

The pipelining of 1.2.21 also gave wrong results. MariaDB 11.8.6, statement A cached, the first use of statement B between two executes of A, 3 of 3 runs each:

Client	Flag	Result
1.2.21	unset	1 of 4 correct, 3 reject with ERR_MYSQL_UNEXPECTED_PACKET
1.2.21	set	2 of 4 correct, 2 queries resolve with the rows of each other
1.2.22	unset	4 of 4 correct
this branch	unset and set	4 of 4 correct

Who sets the flag. In #21403 the author of the flag wrote: "You can also set the environment variable BUN_FEATURE_FLAG_DISABLE_SQL_AUTO_PIPELINING=1 if that's easier". A comment on #33985 names the flag as a way around a Postgres hang. Both users run Postgres. A search for the flag name finds 10 issues and pull requests. None reports the MySQL hang. The flag is in no page under docs/.

Postgres. A proxy in front of PostgreSQL holds the replies. Three queries on a cached statement are queued together. With the flag unset the client writes 3 Bind messages in one write. With the flag set it writes 1. The result is the same without and with this change.

Self-review. 11 concerns from a review of the diff.

Concern	Result
The held-reply test used the exit of the child as its barrier (2 concerns). On Windows a process exit resets the connection.	Changed. The mock ends the connection.
The text reported numbers for test rows that were replaced later.	Each number here is from the rows in this PR, or it names its rows.
The text said that the order test passes on 1.2.21 with the flag set. That pass was chunk timing.	Corrected, tables above. The counter that gave that pass is removed.
"One command at a time" is a side effect of #22619 that nobody decided (5 concerns). The tests made it a contract for the flag-unset state.	#44027 filed, and the comment above the held-reply test links it. That test runs in both flag states, because the review of this PR asked for both. Question in Downsides.
The reach statement did not say that a maintainer told users to set the flag (2 concerns).	Added, with the count of reports.

From the author's own checks: one failure of the order test on Windows under CPU load has no proven cause. See Windows.

Review of this PR. 4 findings. The reviewers marked 3 as optional and 1 as minor.

Finding	Result
In MySQLRequestQueue::advance the continue after a pipelined write cannot run.	Correct, and true since v1.2.22. Not changed here. #44027 has the list of the code that only pipelining needs.
The new tests set a test timeout of 90 s. The reviewer asked for the default and for a short limit for the child.	The test timeouts are removed. The child keeps its 60 s, as in the two mock tests below these: a short limit fails a correct build on a slow machine (Tests).
The held-reply test ran with the flag set only, and its comment said that the flag promises one command at a time.	The test runs in both flag states. The comment says that the adapter does not read the flag.
The fixture of the real-server test did not set allowPublicKeyRetrieval. Over plain TCP the client refuses the full authentication of caching_sha2_password without it (MySQLConnection.rs:831).	The fixture sets it, as getOptions() does. The case was not run: the test machine has no MySQL 9 server. On CI the child passed in 4 builds, probably because a test before it had filled the cache of the server.

Measurements. Release x64 builds of 29d9638 without and with this change, flag unset. The branch was rebased after these builds and the source hunk did not change. They were not repeated on the new base.

• Readers of BUN_FEATURE_FLAG_DISABLE_SQL_AUTO_PIPELINING in src/: 3 -> 2 lines (git grep). The declaration and the Postgres reader stay.
• Flag getter calls on the MySQL path: JSMySQLQuery::run 1 -> 0, MySQLConnection::advance 1 -> 0. Callers in the binary: 4 -> 2, both Postgres (llvm-objdump).
• Gate of the Prepared arm: true path 34 -> 14 instructions, false path 22 -> 2 (static walk, llvm-objdump).
• Instructions that JSMySQLQuery::run executes for one cached prepared query, callees included: 10884 -> 10870 (gdb stepi, mode of 12 calls).
• Flag getter calls: 1001 sequential prepared queries 1002 -> 0, 303 queries in batches of 3: 807 -> 0 (gdb breakpoint hit count).
• Symbol sizes: JSMySQLQuery::run 9740 -> 9857 B, MySQLConnection::advance 1422 -> 1369 B. No other symbol changes size. The .text section is 58301116 B in both builds (nm -S, size -A). run is larger and executes fewer instructions: the disassembly shows other register assignments and another block layout.
• Syscalls for 40001 prepared queries against an in-process mock, without vs with the change: sendto 80009 vs 80009, recvfrom 80010 vs 80010, epoll_wait 80012 vs 80012, read 34 vs 34, write 2 vs 2 (gdb catch syscall, hit count / 2). strace is not installed on the machine.

Tests.

• Real-server test, flag set and unset: each kind of query gets its own rows. It includes the mixed batch from the table above. Its fixture, run by hand against MariaDB 11.8.6: the debug build gives the expected output in both flag states, and 1.4.3-canary.1 gives it with the flag unset and no output with the flag set.
• Order test, flag set and unset: the mock sees PREPARE, 4 EXECUTE, QUERY in queue order.
• Held-reply test, flag set and unset: the mock never answers EXECUTE b. The child prints a line when it has flushed what it queued, and the mock then ends the connection. The client closes its side and writes nothing more (on_end, on_close in JSMySQLConnection.rs). So the mock has every command when the connection has closed. 1 of the 3 queued EXECUTE commands arrives.
• Fail before, release build without the source change (1.4.3-canary.1, 367d939): the 2 flag-set mock rows fail and the 2 flag-unset rows pass. The child hangs after PREPARE, so a local run stops the row at its limit of 5 s. With a limit of 90 s, as on CI, the child is killed after 60 s and the assertion shows that the mock got PREPARE and no EXECUTE. An earlier version of these tests failed in the same rows on the debug build with src/ from main. The real-server test body fails its flag-set row on 1.4.3-canary.1 against MariaDB 11.8.6.
• The assertions can fail. With is_ready_for_query removed from can_pipeline in a local build, the held-reply test fails: EXECUTE c and EXECUTE d are behind the held EXECUTE b. With 1.2.21 as the child, the real-server test fails in both flag states: mixed is ["l", null, null, "o"] with the flag set.
• Release build on Linux, an earlier version with 3 mock rows: 300 of 300 runs pass, 50 ms for each row.
• A slow machine. The tests use the default timeout: 5 s in a local run, 90 s on CI and 270 s on the ASAN lane (scripts/runner.node.ts:1165). The test machine has 128 CPUs at a load average of 380 to 830, and the tests get 8 of them. Debug build there, on the day of the last push: the file passes in 8 of 13 runs, and a row takes 1.8 to 6.5 s. One day earlier the file passed in 1 of 8 runs, and a row took 3.5 to 16 s. Each failure is a row at the 5 s limit. The older test "binary TIME with a very large days field" failed in 3 of those 8 runs. Release build under that load, 4 children at a time: the children of these tests need up to 11.1 s in 2400 runs, with 0 wrong commands. A child that does not use Bun.SQL needs up to 13.4 s in 1200 runs. So a slow run is a machine with no free CPU.
• Each fixture sets connectionTimeout to 100 s, longer than the 60 s that the test gives the child. So a machine that is too slow gives one kind of failure.
• CI on the head commit eb4546b (build 121143): 181 of 181 jobs pass. The job logs of 4 lanes show the counts of this file. Debian 13 x64 and its ASAN lane run 324 tests, against the MySQL, MySQL 9 and MySQL with TLS containers. Windows 2019 x64 and macOS aarch64 run the 6 mock tests. Main has 314 and 2.

Windows. Windows Server 2019 x64, 16 vCPUs. "Load" is 48 busy loops that were checked to be alive.

Tests of this PR, on the CI build of this branch (bc4f8d9, the same source change), both flag states:

Condition	Order test, flag set	Order test, flag unset	Held-reply test
Idle	20 of 20	20 of 20	20 of 20
Load, one test process	180 of 180	180 of 180	180 of 180
Load, two test processes	420 of 420	420 of 420	420 of 420

The last 660 of these runs have the connectionTimeout option.

An earlier version of these tests, on the stock canary f063852. The canary has the defect, so its flag-set rows hang. That version counted the commands without an answer and ran the held-reply fixture with the flag unset too:

• Idle: 200 of 200 for the order test and for the held-reply fixture, flag unset.
• Load, one test process, flag-unset rows only: held-reply fixture 570 of 570, order test 769 of 770. The output of the one failure was not kept, so its cause is not proven.
• Control under the same load, the older test "binary TIME with a very large days field" from the same file: 220 of 220.
• Heavier load with the output kept: two test processes, and the flag-set rows ran too and hung for 60 s each. The flag-unset rows then took a median of 47 to 51 s. They failed in 21 of 174 runs. In 19 the child did not finish in the 60 s that the test gives it (exit code 143). In 2 the 30 s connection timeout of the client ended the child during session setup (ERR_MYSQL_CONNECTION_TIMEOUT, no command on the wire). No failure has a wrong command or an extra command.

So each failure that could be read is a timeout on a machine with no free CPU. That is the probable cause of the one failure without output. It is not proof.

First version of the held-reply test (exit of the child as the barrier, not in this PR): the mock gets ECONNRESET in 300 of 300 runs. Under load the mock does not get the last command in 4 of 300 runs.

Other suites (MariaDB 11.8.6, debug build)

• On 37da174: sql-mysql.test.ts and sql-mysql-queued-query-failure.test.ts, 23 of 23 pass.
• On 9730163: the files sql-mysql-queued-query-failure, sql-mysql-cached-error, sql-mysql.transactions and sql-mysql.helpers with the flag set: 44 pass and 2 fail. With the flag unset: the same.
• On 29d9638: all 20 test/js/sql/sql-mysql*.test.ts files, 55 pass, 1 skip and 2 fail, flag set and flag unset.
• The 2 failures are in sql-mysql.transactions.test.ts. They compare MySQL error text with MariaDB error text, and they fail on 1.4.3-canary.1 too.
• postgres-prepared-pipeline-reorder.test.ts and postgres-simple-query-pipeline.test.ts: 7 of 7 pass, flag set and flag unset. sql.test.ts did not run locally.
• test/internal/source-lints/: 196 of 196 pass.

Left open

• MySQL: query writes can bypass queued-but-unwritten requests in the native request queue #32005, 6 pool shapes and 1 transaction shape of a query dispatched during bind: with this change the flag-set result equals the flag-unset result in 7 of 7, and the flag-unset result equals the result without the change in 7 of 7. No connection hangs. At most 2 commands are written with no reply between them, with and without the change.
• Bun.SQL MySQL: the docs say that queries are pipelined, the adapter writes one command at a time since v1.2.22 #44027: the docs say that MySQL pipelines queries. This PR does not edit the docs and removes no code that only pipelining needs.
• sql.close({ timeout: 0 }) waits for pending queries: with a 2 s query pending it returns after 2006 ms. packages/bun-types/sql.d.ts says that it closes at once. Bun.SQL: close({ timeout: 0 }) does not close immediately when queries are pending #32038 reports it and sql: close({ timeout: 0 }) closes at once (gate on presence, not truthiness) #33740 has a change for it. Not changed here.
• Replace bun_core::hint::cold with core::hint::cold_path #39601 edits a comment line inside the deleted block. The conflict is trivial.
• test(sql): the fixture of "process should exit when idle" does not set allowPublicKeyRetrieval #44092: test/js/sql/sql-idle-exit-fixture.ts does not set allowPublicKeyRetrieval. By the code, the older test "process should exit when idle" then needs a test before it to fill the authentication cache of a MySQL 9 server. Not run and not changed here.

[robobun]

^{Updated 10:47 PM PT - Sep 26th, 2026}

✅ @robobun, your commit eb4546bf0f8ad67135877f3704fa5fde08191fe2 passed in Build #121143! 🎉

---

🧪   To try this PR locally:

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That installs a local version of the PR into your bun-44022 executable, so you can run:

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[robobun]

Status

How to reproduce. MariaDB 11.8.6 or any MySQL server. Save the script as flag.mjs and run BUN_FEATURE_FLAG_DISABLE_SQL_AUTO_PIPELINING=1 bun flag.mjs.

import { SQL } from "bun";
const sql = new SQL({ url: "mysql://root@127.0.0.1:3306/bun_sql_test", max: 1 });
console.log(await sql`SELECT 1 AS v`.simple());
console.log(await sql`SELECT ${1} AS v`);
console.log(await sql`SELECT 3 AS v`.simple());
await sql.close();

Build	Result with the flag set
1.2.21	prints three results
1.2.22, 1.3.0, 1.4.3-canary.1, main	prints one result, then the process never exits
this PR	prints three results

Without the flag each build prints three results. A mock server shows the cause on the wire: the client sends COM_STMT_PREPARE, gets the answer, and never sends COM_STMT_EXECUTE.

CI. Green on the head commit.

• Build 121143, commit eb4546b: 181 of 181 jobs pass.
• The test file of this PR runs 324 tests on Debian 13 x64 and on its ASAN lane, against the MySQL, MySQL 9 and MySQL with TLS containers. It runs the 6 mock tests on Windows 2019 x64 and on macOS aarch64. These counts are from the job logs of those 4 lanes.

Open for a maintainer.

• The PR body has a question: is one command at a time the contract of the MySQL adapter? Bun.SQL MySQL: the docs say that queries are pipelined, the adapter writes one command at a time since v1.2.22 #44027 has the measurements for it, and the list of the code that only pipelining needs. The review of this PR asked for that cleanup. It depends on the answer.
• One test run failed on Windows under CPU load, 1 of 770, and its output was not kept. Its cause is not proven. On the CI build of this branch the tests of this PR pass 1800 of 1800 under the same load. The PR body has the runs.

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Walkthrough

MySQL pipelining eligibility no longer checks the disable-auto-pipelining feature flag. Integration tests cover query execution with both flag states, command ordering, and queued-request rejection after connection closure.

Changes

MySQL auto-pipelining

Layer / File(s)	Summary
Flag-independent eligibility and query coverage src/sql_jsc/mysql/MySQLRequestQueue.rs, test/js/sql/sql-mysql.test.ts	Pipelining eligibility no longer checks the disable-auto-pipelining flag. Child-process tests exercise query paths with the flag set and unset.
Command ordering and stalled execute handling test/js/sql/sql-mysql.test.ts	A mock server records MySQL commands and can leave an execute unanswered. Tests check command order and queued-request rejection after connection closure.

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to eb454

The fixture now has the authentication option needed to exercise the MySQL regression, and no remaining merge-blocking issue is established.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely identifies the main change: MySQL no longer reads DISABLE_SQL_AUTO_PIPELINING in can_pipeline.
Description check	✅ Passed	The description explains the problem, fix, scope, unresolved considerations, and extensive verification results. It does not use the template headings exactly, but it provides the required information…

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[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Additional findings (outside the current diff — GitHub can't attach inline comments there):

• 🟣 src/sql_jsc/mysql/MySQLRequestQueue.rs — Maintainers reading can_pipeline after this merge will believe MySQL still pipelines, but no path can write two commands per round trip. advance sets is_ready_for_query false at MySQLRequestQueue.rs:182 and then asks can_pipeline at :190, which requires it true at :68, so the continue at :192 is unreachable and every queued prepared query costs one full round trip. The PR deletes the only knob that named this behaviour and leaves the dead branch and pipelined_requests bookkeeping in place. Fix: either delete the unreachable branch and the pipelined_requests counter in the same PR, or move the is_ready_for_query clear so that a batch of cached executes really pipelines; either way the docs claiming MySQL pipelines must match.

  Why this was flagged

  Trigger: any MySQL workload that queues several cached prepared queries at once (Promise.all of executes), on every connection at production rate. In advance, src/sql_jsc/mysql/MySQLRequestQueue.rs:182 sets is_ready_for_query false immediately before the can_pipeline call at :190; can_pipeline at :68 returns false whenever that flag is false, so the pipelined continue at :192 never executes and :195 always breaks. Each queued execute therefore waits for the previous reply, one round trip per query, while pipelined_requests at :187 is still incremented and offset arithmetic at :147-154 still assumes several may be in flight. The base commit had the same unreachable branch, but this PR removes the flag branch on the grounds that the adapter already serialises writes, cementing the dead code as the design without deleting it; the repository's docs say MySQL pipelines. The dismissal argued pre_existing and did not weigh that the change makes the dead branch the only remaining pipelining path. Remedy: delete the dead branch and counter, or restore real pipelining and let the flag gate it.

  Verification: pre-existing; acknowledged in diff: the PR description states "Since v1.2.22 ... the adapter writes one command at a time" and files #44027 / asks maintainers whether one-command-at-a-time is the contract, and that claim matches the code. Mechanism verified in /home/claude/bun/src/sql_jsc/mysql/MySQLRequestQueue.rs: advance() does queue_ref.is_ready_for_query.set(false); at line 182, then…

[robobun]

Reply to the review of 215e3f6598, the finding on MySQLRequestQueue::advance.

The finding is correct. This PR does not change that behavior, and #44027 tracks it.

• Mechanism. advance clears is_ready_for_query (MySQLRequestQueue.rs:182) and then calls can_pipeline (:190), which reads that field first (:68). So the continue at :192 cannot run, and each queued execute waits for the reply before it. This is the state since refactor(MySQL) #22619 (v1.2.22), with and without this PR.
• Tracker. Bun.SQL MySQL: the docs say that queries are pipelined, the adapter writes one command at a time since v1.2.22 #44027 asks a maintainer if one command at a time is the contract. It names both remedies. The first is to delete the code that only pipelining needs and to correct the two lines in docs/runtime/sql.mdx. The second is to implement pipelining that keeps the order of the replies and to give the flag a MySQL reader again.
• Why this PR picks neither. The choice between them is that decision. The delete removes the pipelined flag, the pipelined_requests counter and can_pipeline in 5 files. That is the wrong direction if pipelining is intended. The restore is a feature: Bun 1.2.21 pipelined and gave wrong results for a mixed batch (measurements in Bun.SQL MySQL: the docs say that queries are pipelined, the adapter writes one command at a time since v1.2.22 #44027).

I added the list of the code that only pipelining needs to #44027.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

[coderabbitai]

Actionable comments posted: 1

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In @test/js/sql/sql-mysql.test.ts:
- Line 206: Update the child fixture’s Bun.SQL configuration for the plain MySQL
connection to enable public key retrieval, matching the option used by
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[claude]

Code review found no issues

No high-confidence issues detected in this change.

[claude]

I reviewed this push and found no bugs; the points from my earlier review are addressed. A maintainer should still weigh in on the question the description raises: after this change MySQL ignores BUN_FEATURE_FLAG_DISABLE_SQL_AUTO_PIPELINING while Postgres honours it, which is a contract decision rather than something code review can settle.

What was reviewed:

• Traced the deleted branch: the Prepared arm in src/sql_jsc/mysql/MySQLQuery.rs:343 writes COM_STMT_EXECUTE only when can_pipeline() is true, so the flag branch blocked every prepared execute; the remaining predicate is unchanged.
• Confirmed the remaining readers (src/bun_core/env_var.rs:271, src/sql_jsc/postgres/PostgresSQLConnection.rs:1573) are Postgres-only and untouched.
• Checked the new tests: both flag states run in fresh subprocesses, pipes are drained concurrently, the held-reply test now runs both states with a corrected comment, and the 60_000 spawn timeout matches the existing mock tests in the same file.

Extended reasoning...

The diff removes a 10-line early return in MySQLRequestQueue::can_pipeline that consulted BUN_FEATURE_FLAG_DISABLE_SQL_AUTO_PIPELINING, and adds three subprocess-based tests (one docker-gated against a real server, two against an in-process mock) covering both flag states. No security-sensitive surface is touched; the change affects only whether a MySQL prepared-statement execute is ever written when the flag is set. The source change is small and provably correct against the call site, and the two nits from the previous review are addressed in this push. Deferring rather than approving because the PR explicitly asks a maintainer whether one-command-at-a-time is the MySQL contract and whether the flag should keep a MySQL reader, which is a product decision outside code review.

[robobun]

@robobun wake up!!

[Jarred-Sumner]

@robobun wake up!!

[robobun]

@Jarred-Sumner I am here. This PR is ready for your review.

• CI: green on eb4546b, build 121143, 181 of 181 jobs. The new tests ran against the MySQL, MySQL 9 and MySQL with TLS containers.
• Change: it deletes 10 lines in MySQLRequestQueue::can_pipeline. With BUN_FEATURE_FLAG_DISABLE_SQL_AUTO_PIPELINING=1 those lines made each prepared MySQL query hang.
• Decision for you: after this change the MySQL adapter does not read the flag. Today that changes no behavior, because the adapter writes one command at a time since v1.2.22. Bun.SQL MySQL: the docs say that queries are pipelined, the adapter writes one command at a time since v1.2.22 #44027 asks if that is the contract.

If MySQL may ignore the flag, the PR can merge as it is. If you want a different change, tell me which one.